This invention relates, in a numerically controlled (NC) machine tool, to a method for detecting the occurrence of an abnormality in the cutting tool and for deciding an appropriate retreat path for the cutting tool to a tool replacement position and a compatible return path to the workpiece.
In the prior art, when an abnormality occurs in the cutting tool of NC machine tool during its automatic machining operations, an exchange of the cutting tool is conducted at a predetermined place to which the cutting tool is manually transferred after the operation of the NC machine tool has been stopped. To resume the machining operations on a workpiece, both the cutting tool and the controlling program for the NC machine tool must be manually brought back to the positions corresponding to the top of a block of cutting tool path data, during which block the NC machine tool has been stopped. In some instances, only a full restart is possible. Such manual operations require that a great deal of time and effort be devoted to the tool exchange. Additionally, there is a high probability that the cutting tool with inadvertently come into contact with the workpiece during the retreat and return movements, causing further damage to the workpiece, which may require discarding the damaged work.
Such inadvertent damage to the workpiece may be avoided by choosing the retreat and return paths in accordance with the situation existing when the abnormality arises. One potential solution established by the prior art is to memorize all paths taken by the cutting tool before the abnormality is sensed and a retreat command signal is generated and the path is retraced in reverse. However, such systems become complicated and cumbersome, and may cause scratching of the surface of the workpiece during the reverse operation. Further, it is then difficult to restart the machining except from the beginning.
In another example, a system is so constructed and governed so as to automatically retreat and then return the cutting tool over a predetermined retreat and return path when commanded by an abnormality signal. However, in such a system, the retreat and return paths of the cutting tool are conducted in the same manner regardless of the machining operation being performed and without consideration of the size and shape of the workpiece. Operations, such as external diameter machining, edge face machining, and internal diameter machining, each require unique retreat and return paths to be followed by the cutting tool in order to avoid contact between the cutting tool and the workpiece. The prior art does not appear to take such machining mode differences into consideration.
Several methods for detecting an abnormality in the cutting tool are shown in the prior art. One such method is to detect the electric current of the main spindle motor of the NC machine tool and to judge, by an increase in that motor current, that an abnormality has occurred in the cutting tool. A second method is disclosed in which the vibration of a portion of the NC machine tool is picked up by appropriate sensors and the judgment is made on the basis of the amplitude of the vibration. Neither the the main spindle motor current nor the vibration amplitude of the machine tool consistently represent an indication of cutting tool abnormality. Still another method has been proposed in which the force applied to the cutting tool during the machining operation is used for abnormality judgment. However, in this method, a sensor must be provided in close proximity to each cutting tool in order to sense the force applied to the cutting edge of the cutting tool. By being exposed to the environment in the vicinity of the cutting operation, the durability of the sensor and the inherent problems of the lead wire connections between the sensor and that portion of the NC machine tool utilizing the sensor output gives rise to reduced reliability.